A gas turbine engine may be used to power various types of vehicles and systems. A gas turbine engine may include, for example, five major sections: a fan section, a compressor section, a combustor section, a turbine section, and an exhaust nozzle section. The fan section induces air from the surrounding environment into the engine and accelerates a fraction of this air toward the compressor section. The remaining fraction of air induced into the fan section is accelerated through a bypass plenum and exhausted through the mixer nozzle. The compressor section raises the pressure of the air it receives from the fan section and directs the compressed air into the combustor section where it is mixed with fuel and ignited. The high-energy combustion products then flow into and through the turbine section, thereby causing rotationally mounted turbine blades to rotate and generate energy. The air exiting the turbine section is exhausted from the engine through the exhaust nozzle section.
In some engines, the compressor section is implemented with one or more axial and/or centrifugal compressors. A compressor typically includes at least one rotor blade that is rotationally mounted on a hub within a casing. The portion of casing in closest proximity to the rotor blades is referred to as the end wall. From a high pressure efficiency perspective, it is advantageous to minimize the distance between the outer tips of the fan blades and the end wall. However, in some conventional engines, minimizing this distance may increase the likelihood of a stall condition. Engine stall is a phenomenon that occurs as a result of certain engine operating conditions and, if not properly addressed, may adversely impact engine performance and durability.
During operation of the compressor, stall occurs when the stream momentum imparted to the air by the blades is insufficient to overcome the pressure rise across the compressor to result in a reduction in airflow. The compressor stall may propagate through several compressor stages, starving the gas turbine of sufficient air to maintain engine speed. This decreases the turbine's ability to create power, further reducing the output of the engine. To avoid stall, operating limits may be placed on the engine to define a safe operating range in which stall is unlikely. The difference between the safe operating limits and a normal or desired operating condition is often referred to as the stall margin. As in many systems, greater efficiency and overall performance are achieved at higher operating conditions, and thus, to that extent, some conventional compressors may sacrifice engine efficiency to obtain safer operating conditions. To maintain adequate stall margin, the compressor must either operate in a less than optimally efficient manner or mechanisms may be devised to extend the stable operating range of the compressor.
Accordingly, it is desirable to provide casing treatments that increase the stall margin while not adversely affecting engine performance. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.